Circle Size Optical Illusion

Which red circle is larger?

When we look at a two dimensional object, our eyes automatically try to convert it to three dimensions, because we live in a 3D world.

In this optical illusion, since the upper tire (gray circle) is smaller, we assume it is farther away than the lower tire. The inner white circles are the same size, but since the upper white circle covers, proportionally, more of the gray area, we assume it is larger.

Variations of this type of perspective optical illusion are very common.